The present invention relates generally to differential mechanisms, and more particularly, to such mechanisms of the type commonly referred to as "locking differentials".
Differential gear mechanisms of the type to which the present invention applies are broadly referred to as "limited slip differentials"0 and typically include a clutch pack which is operable to limit or retard differentiating action between the output gears (side gears). More specifically, however, the present invention is intended for use on limited slip differentials of the type referred to as "locking differentials", and will be described in connection therewith. In a locking differential, means are provided for engaging or locking the clutch set, rather than permitting it to slip, to substantially reduce the amount of differentiating action permitted between the side gears.
Locking differentials of various types are now generally well known in the art, including both interaxle lockers, and inter-wheel lockers. Inter-wheel locking differentials may be applied either to conventional rear-wheel drive vehicles, or to the more recent front-wheel drive vehicles. Although the present invention may be utilized to advantage in any of the above-described types of locking differential, it is especially advantageous when applied to an inter-wheel, front-wheel drive vehicle, and will be described in connection therewith.
As is now quite well known to those involved in the design of front-wheel drive vehicles, especially the "compact" and "subcompact" automobiles, one of the major problems involves the limited amount of space available for the various engine and drive train components and accessories. The lack of space becomes especially difficult in the case of a component which is "optional" to the vehicle purchaser, and which is larger than the standard component being replaced. Such is the case with a locking differential replacing a conventional, open differential.
In a locking differential, the space problem is conpounded by the dimensional limitations imposed in regard to both the axial length and the gear case outside diameter. For example, the location of adjacent drive train components and accessories may result in a specified maximum case diameter, over at least a major portion of the axial extent of the locking differential. In prior art locking differentials of the type illustrated in U.S. Pat. No. Re.28,004, assigned to the assignee of the present invention, there is included a lockup means for locking up the differential gear set, and an actuating means for actuating the lockup means. The actuating means includes a rotatable flyweight mechanism and a latch mechanism which is movable between an operative position and an inoperative position. In the operative position, the latch mechanism engages the flyweight mechanism to prevent rotation thereof and actuate the lockup means, and in the inoperative position, the latch mechanism is incapable of engaging the flyweights. Typically, the inoperative position occurs when the rotational speed of the differential gear case exceeds a predetermined limit. Normally, with no differentiation occurring between the side gears, the vehicle speed is a direct function of the gear case rotational speed.
As is well known to those skilled the art, the performance of an actuating means of the type to which the present invention applies can be improved by increasing the size of the flyweight mechanism, and by increasing the weight of the latch mechanism. However, in prior art locking differentials, the flyweight mechanism and the latch mechanism have been disposed circumferentially adjacent each other, and in a locking differential for a front-wheel drive vehicle, requiring a reduced case diameter, there would be insufficient space to utilize the prior art arrangement, with the flyweight and latch mechanisms adjacent each other, unless the size and weight of the mechanisms would be reduced, which would adversely effect the performance.
Accordingly, it is an object of the present invention to provide a differential gear mechanism of the locking differential type which makes it possible to reduce the gear case diameter without the need to reduce the size of the flyweight mechanism or the weight of the latch mechanism.
It is a more specific object of the present invention to provide a locking differential in which the flyweight mechanism and latch mechanism are not disposed circumferentially adjacent each other.
It is another object of the present invention to provide a locking differential mechanism which accomplishes the above-stated objects, without increasing the axial length of the differential gear mechanism.
Performance of a locking differential, and more specifically, of the flyweight and latch mechanisms, is evaluated primarily in terms of "missed engagements," i.e., the number of times that one of the wheels "spins out" or accelerates, without the latch mechanism engaging the flyweight mechanism to initiate locking of the differential gear set. One of the primary causes of missed engagement is the effect of gravity on the weighted portion of the latch mechanism. For example, if the latch mechanism is designed not to engage at case rotational speeds above 200 rpm, the actual maximum engagement speed will vary depending upon the rotational orientation of the differential case at the instant the flyweights move to the extended position. A graph of case rotational speed (maximum speed at which engagement can occur) versus rotational orientation is shaped generally like a sine wave, with the amplitude of the curve indicating the relative effect of gravity on the operation of the latch mechanism. This sine wave may also be considered as a graph of case speed above which lockout of the latch mechanism (i.e., movement of the latch mechanism to an inoperative position) will occur, and therefore, will be referred to hereinafter, for simplicity, as the "lockout" curve. Using the example above, with a case speed of 200 rpm being the selected maximum "lockout" speed, it will next be assumed for purposes of explanation that the locking differential encounters a "spin out" condition, such that one side gear rotates at 0 rpm, and the other side gear begins to accelerate.
If the flyweights are designed to move to the extended position at 100 rpm difference between the side gears, no engagement can occur at case speeds below 50 rpm (the average of the side gear speeds). A line may then be placed on the above-mentioned graph at the 50 rpm level. Evaluation of this graph indicates that engagement will not occur at case speeds above the "lockout" curve, nor below the 50 rpm line. If the "lockout" curve dips below the 50 rpm line, over a range of case orientations, the indicated result is that engagement cannot occur within that particular range of rotational orientations, resulting in a spin out or acceleration. Therefore, in designing a flyweight mechanism and latch mechanism for a locking differential, it is one objective of those skilled in the art to reduce the amplitude of the above-described "lockout" curve, thus reducing the range of case orientations in which a misses engagement can occur.
The attempts to reduce the amplitude of the engagement curve are related to the previously-described size and space problems involved in the design of locking differentials for front-wheel drive vehicles. It has been found that the amplitude of the lockout curve is related to the geometry of the latch mechanism, and more specifically, to the center of gravity (CG) of the weighted portion of the latch mechanism. One of the factors involved in the radius (R), i.e., the distance from the axis of rotation of the differential to the CG of the weight, when the latch mechanism is in the operative position, i.e., the position in which engagement of the flyweights can occur. As R increases, the amplitude of the curve decreases. Therefore, for a given locking differential design, and with all other factors remaining constant, the frequency of missed engagements can be reduced by increasing R, which theoretically may be accomplished by moving the latch mechanism further outward radially on the differential. However, because to the difficulty of finding sufficient space for the latch mechanism in prior art locking differentials, it has already been customary for the flyweight mechanism and latch mechanism to be placed in an opening defined by the wall of the case, far enough inside the case diameter to permit the latch mechanism to pivot from the operative position to an inoperative position, without any portion of the mechanism moving outside the case diameter.
Accordingly, it is an object of the present invention to provide a latch mechanism for a locking differential which is capable of reducing the amplitude of the "lockout" curve, without the need to increase the case diameter.
It is a more specific object of the present invention to provide a latch mechanism for a locking differential having an increased radius to its center of gravity without causing an increase in the case diameter.